Ceramics are generally known to be brittle and liable to damage.
The present invention relates to superconducting ceramics including htsc ceramics. In particular the present invention relates to htsc ceramics based on bismuth-(lead)-strontium-calcium-copper-oxides (referred to as Bi2223 and Bi2212, and PbBi2223 and PbBi2212, respectively) and rare-earth element(s)alkaline earth element(s)-copper-oxides (referred to as YBCO 123 and YBCO 211). Such superconducting ceramics as well as composition and structure thereof and process for preparation thereof are well known.
Tube-shaped hollow bodies made of such superconducting materials are widely used in superconducting applications. It is well known to use tube-shaped superconducting hollow bodies made of, for example, Bi2212 in htsc current limiters and current leads to low temperature devices.
For example, current limiters based on Bi2212 tubes are disclosed in Patent Application No. 0 573 798 and 0 524 442 which by reference is incorporated herein.
Such htsc tubes are obtainable by a well known melt casting process, preferably by centrifugal casting process as disclosed for example in European Patent Application No. 0 462 409 also incorporated herein by reference.
In general, for use in current limiters and current leads, respectively, both ends of the htsc tubes are provided with contact surfaces that are preferably formed of silver.
Usually, metallic elements are attached to the contact surfaces, said metallic elements serving as connection of the htsc tube to the current supply.
There exists the problem that by the weight of the metallic elements, in particular in addition with the weight of current supplies connected thereto, a bending moment is generated on the htsc tube. Further, when supplied with current on both ends of the htsc tube moments are acting caused by Lorentz forces. Due to these forces acting on the htsc tube damage or even breakage of the htsc tube can occur.
For stabilising the htsc tubes against these forces it is known to introduce a reinforcing pipe made of non-conducting or only poorly conducting material, e.g. glass fibre reinforced plastics, into the htsc tube.
For stabilization purposes the reinforcement pipe is fastened to the metallic elements and to the surface of the htsc tube, respectively, for example by sticking, screwing and soldering, respectively. In particular, the inner surface of the htsc tube and the outer surface of the reinforcing pipe have to be connected tightly to each other.
In operation the thus obtained composite is subjected to significant temperature changes resulting in volume changes of the components of the composite. In order to avoid damage of the htsc ceramics due to different volume changes during temperature change it is necessary to adjust the thermal expansion coefficients of the material used both in radial and axial direction.
There is a disadvantage that selection of materials with suitable thermal expansion coefficients are restricted by the material requirements of the specific application.
Furthermore, during cooling not only deformation of the reinforcing pipe has been observed but also generation of radial cracks in the htsc tube.